As described, for example, in U.S. Patent Publication No. 2007/0132408 (“the '408 Publication”), the disclosure of which is incorporated by reference herein, light in the spectral region below about 400 nanometers wavelength, normally referred to as “ultraviolet” or “UV” light, and particularly between about 100-200 nanometer wavelength, commonly referred to as the “vacuum ultraviolet” or “VUV” region of the spectrum, can be generated by forming excimers. Excimers are transient molecules composed of atoms that do not normally combine with one another. One or more of the atoms constituting an excimer is in an excited state, i.e., a state in which the atom has been momentarily promoted to a higher energy state as, for example, by promoting one or more electrons to higher energy orbitals. The excimer molecule as a whole is also in an excited state, and will ultimately decay to yield the constituent atoms. Decay of certain excimers is accompanied by emission of ultraviolet light.
Elements commonly referred to as “rare gases” or “inert gases,” such as helium, neon, argon, krypton, and xenon, which normally exist only as isolated atoms, can form excimers. For example, diatomic rare gas excimers such as Ar2*, Kr2*, and Xe2* will emit radiation in the VUV range upon decay to the constituent atoms. Other excimers can be formed by combination of a rare gas and a halogen atom. For example, excimers such as ArF* and XeCl* emit light at about 193 nm and about 308 nm, respectively.
As described in the '408 Publication, excimers of this type can be formed by applying radiofrequency (“RF”) energy between a pair of electrodes disposed in a chamber containing the gas. In one embodiment, the UV light source according to the '408 Publication includes electrodes disposed at a small spacing, as for example, about 1 mm or less, within a chamber. A gas mixture containing a rare gas, a halogen, and typically also containing a diluent such as helium, neon, or argon which does not constitute a constituent of the excimer to be formed, is supplied within the chamber. RF energy is applied between the electrodes so as to form a discharge. Such a light source provides a bright and stable point source for UV illumination. Typically, the chamber includes a window which is transparent to the emitted UV light.
UV sources of this type can be applied in many different industrial and scientific processes, as for example, in photolithographic processes for forming semiconductor devices and other structures of comparable dimensions; in fingerprint detection; in microscopy; and in excitation of certain chemical reactions.
As described in Laroussi, U.S. Pat. No. 6,858,988, an “electrodeless” excimer lamp has a chamber formed from a dielectric material such as glass or quartz, and surrounded by ring-like electrodes. The chamber is filled with an excimer-forming gas mixture. RF energy applied between the electrodes provides an electric field within the chamber, which also creates a plasma within the chamber and leads to formation of excimers. As shown in Laroussi '988, the emitted light is essentially isotropic, and is emitted in all directions through the sides of the chamber for applications such as disinfecting liquids surrounding the chamber. Because the electrodes do not directly contact the excimer-forming gas mixture, the system is referred to as an “electrodeless” discharge lamp.
Despite the considerable efforts in the art devoted to creation of useful UV light sources, still further improvement would be desirable. In particular, it would be desirable to provide a UV light source with very high brightness in a concentrated spot.